A drafting arrangement has the task to draw or draft a delivered fiber strand in order to reduce in this manner the number of fibers in cross-section. During drafting, the fibers have be displaced relative to each other as uniform as possible while overcoming static friction so that, as a result, a fiber strand as uniform as possible and subsequently a harmonious yarn is obtained.
In the case of spinning machines, a distinction is made between preliminary spinning machines and final spinning machines, also called fine spinning machines. Preliminary machines, which include in particular roving frames, produce a roving from a sliver, in particular from a drafter sliver, which roving is processed in the next process stage into a yarn by means of a final spinning machine. Accordingly, preliminary spinning machines comprise a drafting arrangement which drafts a sliver, in particular a drafter sliver, so as to achieve the fineness of a roving. Final spinning machines, in turn, comprise in each case drafting arrangements which draft the roving to the fineness of a yarn. Also known are final spinning machines, the drafting arrangements of which, while omitting the preliminary spinning machine, are designed to draft a drafter sliver in one process stage up to the fineness of a yarn. In literature, the associated drafting arrangements are called high-draft drafting arrangements. Within the context of this invention, all above-mentioned drafting arrangement types, alone or in combination with one of the mentioned spinning machines, shall come within the scope of the present invention.
A drafting arrangement usually comprises a plurality of successively arranged pairs of rollers which each forming a clamping line or a clamping region. The pairs of rollers consist in each case of a bottom roller that is usually driven and an upper pressure roller through which the fiber strand is clamped. The fiber strand is transported through the pairs of rollers to a twist generation. The draft is generated in that the circumferential speed increases in the running direction of the fibers from roller pair to roller pair. The draft between the last two roller pairs in the running direction of the fibers is called main draft; the sections upstream thereof are called break draft. There are spinning machines which have one or two break drafts, while spinning machines having only two pairs of rollers, thus effect only the actual main draft, are not known in practice.
Drawing a part of the fibers is carried out by carrying the fibers along on the roller surfaces or aprons arranged thereabove, for which reason the fibers have to adopt approximately the circumferential speed of the rollers. Transmitting the roller movement to the fibers to be transported takes place through friction.
The roller pair from the clamping point of which the main draft starts contributes to good guidance of the fibers. Here, it is frequently the case that each of the two rollers is supplemented by at least one further deflection element and an apron running about the latter and the rollers. This construction is known as a double-apron drafting arrangement.
An alternative to this is the Kepa drafting arrangement, named after the inventors Kern and Pauen, which has modified this established structure: Bottom roller and cage of the roller pair which initiates the main draft are replaced by a large bottom roller while the top apron and its cage have a particularly long design so that the top apron and the large bottom roller are in contact as long as possible to achieve a good fiber guidance. Like a conventional drafting arrangement, the Kepa drafting arrangement can be implemented as two-, three- or four-roller drafting arrangement. The Kepa drafting arrangement is also used as a high-draft drafting arrangement, whereby the requirements for exact fiber guidance are even higher. Such a drafting arrangement is described, e.g., in the European patent specification EP 0 350 797 B1.
As can be derived from the above descriptions, the drafting arrangement is the centerpiece of a spinning machine because here, the quality of the produced yarn is determined. Changes in the drafting arrangement and to its geometry can have a significant influence on quality. This is certainly one of the reasons that in the sector of drafting arrangements of spinning machines, no fundamental changes have been made for a long time. Thus, the drafting arrangements of ring spinning machines are still driven by common rollers which extend over a plurality of spinning stations or an entire longitudinal machine side. Accordingly, drafting arrangements on one side of a machine have to be operated synchronously. Operating the spinning stations individually is not possible. The ever-increasing length of spinning machines cause torsion problems in the drafting arrangements so that multiple drives have to be used for operation. Thus, shutting down or individually maintaining the drafting arrangements is possible only to a limited extent.